To hear or not to hear: selective tidal stream transport can interfere with the detectability of migrating silver eels in a Tidal River

Abstract

Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels (Anguilla anguilla), so called silver eels. The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study quantifies the impact of these environmental factors on both, migration behaviour and receiver performance to determine possible limiting conditions for detecting tagged eels in tidal areas. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST). It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during increased current velocities, meaning that high migration activity and -speed coincides with reduced detection probability. Consequently, there is a risk that particularly during phases of increased activity, migration activity may be underestimated due to reduced acoustic telemetry performance. To avoid bias in telemetry studies, it is, therefore, crucial to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities.